The long-term goals of this proposal are to increase the understanding of the cellular and humoral events regulating megakaryocyte development. Using a murine mode, we will study the responsiveness of (partially) purified progenitor cells to purified molecules such as IL-3, TSF, or PMA. We also will examine the responsiveness of early (BFU-Mk) and late (CFU-Mk) progenitor cells as well as early differentiated (immature) megakaryocytes to these factor. Purified T cell and macrophage populations will be used to investigate the role of accessory cells and immune networks in megakaryocyte factor production. Finally, synthetic phospholipids and exogenous phospholipase C will be used to continue investigations on the role of phorbol diesters in IL-3 driven megakaryocyte colony formation. Studies on human megakaryocytopoiesis will seek to identify the various progenitor cell subpopulations and auxiliary cells involved in in vitro megakaryocyte development. In particular, we will attempt to identify early high proliferative progenitors and examine the relationship of pluripotent cells (CFU-GEMM) to megakaryocyte lineage-restricted colony forming cells. Also, emphasis will be given to the cellular and biochemical characterization of synergistic co-regulator of megakaryocyte development. This activity predominantly influences human megakaryocyte maturation, in vitro. Importantly, conditioned media containing this activity also stimulates in vivo murine platelet production. Finally, two groups of patients with select platelet disorders (congenital thrombocytopenia and essential thrombocythemia) will be studied to obtain information on megakaryocytopoiesis in disease. The long- term health related aims of this project are to improve our knowledge of the process of platelet production, abnormalities of which result in a spectrum of problems extending from hemorrhage to thrombosis function.